An intraocular lens (IOL) is an artificial lens which is used for implantation into a human eye to replace a natural lens in the human eye which becomes opacified due to the cataract disease, or which is used in a refractive operation to correct vision of the human eye. An intraocular lens usually consists of a circular optic and support haptics disposed on the circumference thereof. The optic of the intraocular lens is directly connected to the support haptics. The optic of the intraocular lens consists of an effective optical area (also called as “the optic body” in the art) and an effective optical area edge. The intraocular lens made of a soft material is often called a foldable intraocular lens and may be folded or rolled to get smaller and then be implanted into a human eye through a smaller incision (from less than 2 mm to 3 mm). The folded or rolled intraocular lens can unfold automatically after entry into the eye.
The flexible foldable intraocular lenses are generally classified into one-piece type and three-piece type according to the engagement manner of the optic and the haptics. In the one-piece type flexible foldable intraocular lens, the optic and the support haptics are integral and made of the same piece of soft material. In the three-piece type flexible foldable intraocular lens, the optic and the support haptics are processed separately and then combined and connected together.
So far, the soft materials for preparing the foldable intraocular lens are generally classified into silicone, hydrophilic acrylate (hydrogel), hydrophobic acrylate, and polymethyl methacrylate (PMMA). The hydrophobic acrylate is currently the most widely used IOL material. It has the advantages such as a high refractive index and a moderate unfolding speed from a folded state. For example, the U.S. Pat. Nos. 4,834,750, 5,290,892 and 5,814,680 disclose several different methods for preparing the IOL from hydrophobic acrylate materials.
The posterior chamber IOL 1 (hereinafter referred to as “IOL”) maintains at a relative position in a posterior chamber capsule bag 12 by means of the interaction force between the support hapics 5 and the capsule bag 12 after being implanted into a human eye. The retraction and expansion of the capsule bag act upon the support haptics, so that the IOL connected to the haptics is pressed or stretched to move forward and rearward along an ocular axis direction D-D′. The optics 2 of the IOL 1 and a cornea 11 of the human eye jointly form a dioptric system and bear about 30% of the refractive power of the human eye, as shown in FIG. 1. In this note, when light enters from a substance into another substance with different optical density, propagation direction of the light will deflect. This phenomenon is called refraction phenomenon. Diopter indicates a magnitude of the refraction phenomenon (refractive power), with a measure unit of diopter (“D” in short). 1D refractive power is equivalent to focusing parallel light rays on a 1-meter focal length. The action of the eye refracting light rays is called refraction. Focal power of light is used to represent capability of refraction, and is also called diopter. The diopter is the lens's refraction intensity for the light rays. The diopter is a measure unit of refractive power and represented as D. When parallel light rays pass through the refraction substance, and the refractive power of the refraction substance at the 1-meter focusing point is 1 diopter or 1D. As for a lens, the diopter refers to the measure unit of a focal power of the lens, e.g., when a focal length of a lens is 1M, the refractive power of the lens is 1D diopter, inversely proportional to the focal length. The refractive power of the lens is F=1/f, wherein f is the focal length of the lens. In the equation, the measure unit of the refractive power is diopter with a symbol of D, a dimension of L−1, 1D=1 m−1.
Those skilled in the art appreciate that imaging quality of the IOL is a factor that must be considered during design of an IOL product.
The IOL, in addition to providing the refractive power to compensate the refractive power of the cornea, needs to correct the cornea and its own various high order aberrations to achieve high-quality imaging quality.
Refractive error is a factor substantially affecting the imaging quality, wherein astigmatism is a common refractive error phenomenon of human eyes and refers to a phenomenon that the eyeball has inconsistent refractive power on different meridian lines, or unequal diopters on the same meridian line, so that parallel light rays entering the eye cannot form a focal point on a retina, but instead form a focal line. Astigmatism is clinically classified into regular astigmatism and irregular astigmatism. The regular astigmatism means that two meridian lines with a maximum refractive power difference are called main meridian lines, and the two main meridian lines are perpendicular to each other. The irregular astigmatism means that astigmatism bending degrees of meridian lines are inconsistent. The regular astigmatism may be corrected through lens.
Among normal population, those whose cornea astigmatism is greater than 1.5D accounts for 15%-29%, which seriously affects people's visual quality. The newest treatment method for cataract with astigmatism is to implant an astigmatism IOL (Toric IOL) in the eye to achieve normal refraction and meanwhile correct cornea astigmatism.
Toric IOL was marketed since 1997 and consecutively ratified by FDA of the United States and the security certification of European Community (EC). The earliest Toric IOL is achieved by attaching a cylindrical surface to the posterior surface of the IOL (the basic surface shape is convex in the front and flat in the rear; the cylindrical surface is directly attached to the posterior surface). Currently a relatively mature Toric IOL adopts a design of a toric surface, which integrates cylindrical surface refraction effect with a spherical surface and an aspherical surface. Typically, Acrys of astigmatism IOL produced by Alcon Corporation of the United States adopts Toric design at the posterior surface of the lens and can correct 1.03D-4.11D astigmatism of human eye cornea; TECNIS Toric series IOL produced by AMO Inc. may correct 0.69D-2.74D astigmatism of human eye cornea. Meanwhile, the improved “L” haptics or “C” haptics are used to improve stability of the lens in the human eye.
Besides, high order aberrations also affect the imaging quality. The high order aberrations mainly comprise spherical aberration and comatic aberration.
In a human eye dioptric system, the spherical aberration is a factor that most affects the imaging quality besides refractive error, and appears particularly obvious when the human eye is in a dim condition with a large pupil (the pupil diameter 4.5 mm-6.0 mm). A radius of curvature of an optical surface with a minimum IOL spherical aberration may be obtained by calculation, and the obtained radius of curvature of the optical surface is related to the refractive index of the IOL material. Table 1 shows radii of curvature of the two surfaces with different refractive indices when the effective optical area is a spherical surface design and when the IOL spherical aberration is minimum. The following equations are used upon calculation:
                                          r            2                                r            1                          =                              n            ⁡                          (                                                2                  ⁢                                                                          ⁢                  n                                +                1                            )                                                          2              ⁢                                                          ⁢                              n                2                                      -            n            -            4                                              (        1        )                                φ        =                                            φ              1                        +                          φ              2                                =                                                                      n                  -                                      n                    ′                                                                    r                  1                                            +                                                                    n                    ′                                    -                  n                                                  r                  2                                                      =                                          (                                  n                  -                                      n                    ′                                                  )                            ·                              (                                                      1                                          r                      1                                                        -                                      1                                          r                      2                                                                      )                                                                        (        2        )            
r1 and r2 are respectively radii of curvature of the IOL anterior and posterior surfaces, n is the refractive index of IOL material, n′ is the refractive index of vitreous body and aqueous humor, and φ1 and φ2 are diopters of the anterior and posterior surfaces. Equation (1) is derived as follows when the spherical aberration equation of the lens achieves an extreme value.
      δ    ⁢                  ⁢          L      0      ′        =            -              1                  2          ⁢                                          ⁢                      n            ′                    ⁢                      u                          ′              ⁢                                                          ⁢              2                                            ⁢          h      4        ⁢    A  
Wherein:
                    A        =                                                            n                +                2                            n                        ⁢            φ            ⁢                                                  ⁢                          ρ              1              2                                -                                    (                                                                                                                  2                        ⁢                                                                                                  ⁢                        n                                            +                      1                                                              n                      -                      1                                                        ⁢                                      φ                    2                                                  +                                                                                                    4                        ⁢                                                                                                  ⁢                        n                                            +                      4                                        n                                    ⁢                  φ                  ⁢                                                                          ⁢                                      σ                    1                                                              )                        ⁢                          ρ              1                                +                                                                      3                  ⁢                                                                          ⁢                  n                                +                1                                            n                -                1                                      ⁢                          φ              2                        ⁢                          σ              1                                +                                                                      3                  ⁢                                                                          ⁢                  n                                +                2                            n                        ⁢            φ            ⁢                                                  ⁢                          σ              1              2                                +                                                    n                2                                                              (                                      n                    -                    1                                    )                                2                                      ⁢                          φ              3                                                          (        3        )            
TABLE 1radii of curvature of the two surfaces with differentrefractive indices when the IOL spherical aberration is minimumRefractiveRadius ofmaterialsindexcurvature30D25D20D15D10D5Dsilicone/n = 1.46r1(mm)5.646.568.2110.8516.2631.97hydrophilicr2(mm)26.9031.4039.4951.8378.35154.89acrylatehydrophobicn = 1.55r1(mm)8.089.7012.1216.1624.2448.49acrylater2(mm)60.7372.9091.16121.58182.43365.00
The spherical aberration of an IOL with a given refractive power and a given refractive index changes in a parabolic form, as shown in FIG. 2. In the graph shown in FIG. 2, the horizontal ordinate ρ1 represents a reciprocal of a radius of curvature of IOL effective optical area anterior surface (the smaller ρ1 is, the flatter the effective optical area anterior surface is), and ρ1 with different magnitudes substantially correspond to the prior-art IOL with different surface shape designs; the longitudinal ordinate δL0′ represents a magnitude of spherical aberration. As can be seen from FIG. 2 and Table 1, the surface shape of the IOL effective optical area 3 will substantially affects the imaging quality. To minimize the spherical aberration (δL0′) to improve the imaging quality, the surface shape of the prior-art spherical IOL is generally convex-flat (i.e. convex in the front and flat in the rear) or double-convex (the effective optical area anterior surface is obviously convex and the effective optical area posterior surface is slightly rearwardly convex), conforming to the surface shape design principle of minimizing the primary spherical aberration in a wholly curving manner in optical design. The types of the radii of curvature of the prior-art IOL anterior and posterior surfaces are approximate to what are shown in Table 1: the posterior surface tends to be flat and the anterior surface is obviously convex, and the anterior surface radius of curvature is universally smaller than the curvature radius of the posterior surface. Clinical implantation results indicate that the convex-flat or obviously forwardly convex effective optical area structure of the spherical IOL achieves a better imaging quality. Therefore, many IOLs choose to adopt these two common surface shape designs so far.
Regarding the case in which the radius of curvature of the effective optical area posterior surface is obviously smaller than that of the effective optical area anterior surface, the IOL with an obviously rearwardly convex effective optical area, upon application, generates a larger residual spherical aberration than the so far universally-used ordinary IOL with flat-convex or slightly rearwardly convex surface shape as mentioned above. As shown in FIG. 2, the design with a small radius of curvature of the IOL effective optical area posterior surface sacrifices part of imaging quality because the difference of radii of curvature of the effective optical area anterior and posterior surfaces makes the obviously rearwardly convex IOL itself have a larger residual spherical aberration. The larger the residual spherical aberration is, the poorer the imaging quality is.
Besides, those skilled in the art should also appreciate that although the prior-art IOL adopting ordinary aspherical (namely, a single aspherical coefficient Q value) surface shape design can compensate spherical aberration, the IOL implanted into the posterior chamber is not always in a perfect central position of human eye posterior chamber, and gets tilted or off-center to some degree and thereby generates the other higher order aberrations besides spherical aberration, typically the comatic aberration. The imaging quality of the prior-art IOL will be reduced due to an error of actual position of the IOL in the eye, and optical performance is extremely sensitive to actual clinical situations.
Posterior Capsule Opacification, the so-called secondary cataract, is a common complication after implantation of the IOL. Posterior capsule opacification is caused by multiplication and migration of the residual lens epithelial cells after cataract surgery to the place between the IOL posterior surface and posterior capsule. The effective optical area of the IOL adopts a sharp right-angle edge design, for example, in U.S. Pat. Nos. 6,162,249 and 6,468,306, which has already been proved a method of effectively reducing posterior capsule opacification because this design can prevent the lens epithelial cells from migrating to the place between the IOL posterior surface and posterior capsule (see the article by Buehl et. al., Journal of Cataract and Refractive Surgery, vol. 34, pages 1976-1985). This sharp right-angle edge design can be implemented on the three-piece IOL more easily because the support haptics are very thin and they are inserted onto the effective optical area. It is more difficult to implement this sharp right-angle edge design on the one-piece IOL because the support haptics are integrally connected with the effective optical area and the support haptics are made of a soft material and need to be produced wider and thicker. To implement the sharp right-angle edge design on the one-piece IOL, the effective optical area needs to have a thick edge and thin support haptics, or the right-angle edge step has a small fall. If the edge of the effective optical area is too thick, an overall size of the IOL will be increased and a small-incision surgery will be made more difficult; if the support haptics are too thin, the action force between it and the capsule is insufficient and the IOL is unstable in the capsule; if the right-angle edge step has a too small fall, it cannot play a role of preventing the migration of the lens epithelial cells.
In an optical design of the prior-art posterior chamber IOL, in order to reduce the spherical aberration and improve the imaging quality, a spherical IOL is generally designed to have an obviously convex anterior surface and a relatively flat posterior surface, with a radius of curvature of the anterior surface being universally smaller than that of the posterior surface. Aspherical IOLs for correcting the spherical aberration and Toric IOL for correcting astigmatism developed subsequently both conform to this design concept. The effective optical area of the prior-art IOL is not rearwardly convex obviously (even in a planar shape), thus a larger gap is left between the IOL posterior surface and the posterior capsule after the implantation of the IOL into the human eye, which causes the position of the IOL unstable and causes posterior capsule opacification after the surgery. Even if the IOL adopts the right-angle edge design (square edge design), when the human eye's ciliary muscle automatically adjusts by retracting or expanding as the eye views far or near, the posterior capsule is driven to move forward and backward under pressure of the vitreous humor, pressure and uneven traction applied by the root area of the IOL support haptics to the posterior capsule bring PCO into the effective optical area edge of the IOL through the flow of the vitreous humor.
At present, secondary cataract has already become a problem that troubles the cataract sufferer and is to be solved urgently. In order to improve stability of a spatial position of the intraocular lens in a capsule bag and to facilitate reduction of an incidence rate of secondary cataract after implantation of the intraocular lens, if the prior-art IOL effective optical area posterior surface adopts a small radius of curvature, it will certainly sacrifice part of imaging quality of the prior-art IOL.
To those skilled in the art, a good IOL design should consider and balance the following factors: to ensure the stability of the IOL in the capsule, reduce the probability of posterior capsule opacification, achieve excellent imaging quality, ensure timely unfolding of the IOL after implantation into the eye, and prevent adhesion of the support haptics and the effective optical area. Therefore, those skilled in the art needs a posterior chamber IOL with an obviously backwardly convex effective optical area, which can improve undesirable imaging quality of the backwardly convex IOL in the prior art.